1. Field of the Invention
The invention is directed to a magnetic resonance apparatus.
2. Description of the Prior Art
Magnetic resonance technology is a known technique for, among other things, acquiring images of the inside of the body of an examination subject. In a magnetic resonance apparatus, rapidly switched gradient fields that are generated by a gradient system are superimposed on a static basic magnetic field that is generated by a basic field magnet. The magnetic resonance apparatus also has a radio-frequency system that emits radio-frequency signals into the examination subject for triggering magnetic resonance signals and that picks up the triggered magnetic resonance signals, on the basis of which magnetic resonance images are produced.
The magnetic resonance apparatus has an examination space containing an imaging volume wherein a region of the examination subject to be imaged is to be positioned for producing magnetic resonance images of that region. The positioning of the region to be imaged in the imaging volume is possible by displacement of a movable support mechanism with the examination subject placed thereon.
The gradient system of the magnetic resonance apparatus includes a gradient coil arrangement that must be rigidly connected to the basic field magnet. To that end, German OS 197 22 481 discloses a magnetic resonance apparatus wherein a basic field magnet has a first surface and a rigidly installed gradient coil system with a second surface, the two surfaces facing toward one another and being spaced from one another. A noise reduction device for damping the oscillations of the gradient coil system and/or for stiffening the gradient coil system is arranged in contact with both surfaces. In one embodiment, the noise reduction device is formed of at least one pillow. Such a pillow preferably comprises an envelope and a core or a filling. In order to achieve an especially good noise-damping effect, the pillow is elastic and/or resilient and/or flexible. In the operating state of the magnetic resonance apparatus, the pillow lies tightly against the first and the second surfaces but is not rigidly connected to these surfaces. As a result, the gradient coil system can be more easily detached for maintenance or replacement (which requires the rigid connection to be dismantled. The pillow has an air-impermeable outside skin that is formed of welded plastic film. The outside skin is composed of PVC film, polyethylene film or some other film that is airtight and weldable. A foamed fill composed of an open-pore cellular material, for example polyurethane foam, is contained in the pillow. The fill of cellular material insures good noise damping as well as providing an adequate elasticity of the pillow when the air pressure in the pillow is approximately the same as the ambient air pressure. The pillow has a connection that can be fashioned as a valve. The pillow is thicker than the distance between the first and second surfaces when the pillow is not introduced into the magnetic resonance apparatus and the valve is open. When, with the valve open, this pillow is introduced into the magnetic resonance apparatus, then the fill of cellular material presses the outside skin against the first and second surfaces, so the gap between these surfaces is completely filled. A pillow with an airtight outside skin and a fill of cellular material can be easily introduced into the gap between the first and second surfaces and be removed therefrom when the air contained in the pillow is pumped out to such an extent that it is compressed to a thinner thickness by the ambient air pressure. In an alternative embodiment of the noise reduction device, the pillow be pumped to a slight over-pressure and/or it can be filled with some other gas or a fluid and/or the inner fill of cellular material can be omitted.
In addition to a rigidly installed gradient coil system, moreover, U.S. Pat. No. 5,185,576 discloses a local gradient coil unit that is combined with a local radio-frequency antenna. The local gradient coil unit with the integrated local radio-frequency antenna is designed for a specific region of the examination subject, for example for the head of a patient. As a result, the local gradient coil unit can be implemented with smaller dimensions compared to the rigidly installed gradient coil system, yielding advantages in view of—among other things—gradient intensities that can be achieved and power demands made on the gradient amplifier that feeds the gradient coil unit. The local gradient coil unit with the integrated local radio-frequency antenna can be secured to the support mechanism such that the local gradient coil unit does not move relative to the support mechanism even during operation of the magnetic resonance apparatus despite the forces that act on it.